This invention relates to key telephone intercom systems and, more particularly, to tone selective or push button key telephone intercom systems.
Key telephone systems of this type commonly utilize voice frequency coded digit transmission schemes based upon dual-tone multi-frequency (DTMF) codes in which a tone pair representing a particular decimal digit station address or portion of a station address is "dialed" or generated at the calling station by depressing the appropriate key pad push button with the calling station telephone set off hook and keyed to an intercom circuit. For single digit codes, only one push button need be depressed; for double digit codes, two push buttons (or the same push button) are depressed in sequence, and so on. Each push button depression produces a tone burst containing DTMF coded signals which are transmitted over the common intercom circuit or channel to a receiver which decodes and identifies the decimal digit or station address represented by the tone signals with respect to an audio tone matrix. According to the so-called 4.times.4 audio tone matrix, each digit is represented by one tone from a high frequency tone group, and one tone from a low frequency tone group. The concurrence of a selected pair of tones, one from each group, represents a decimal digit. The DTMF code is composed of nonharmonically related frequencies in order to protect the system against false-keying by stray signals and voice-generated tones.
Prior tone selective key telephone systems accomplish tone detection by analogue circuitry which, in many practical tone selective key telephone applications, is less than satisfactory from the standpoints of manufacturing costs, performance, size and/or reliability. More specifically, the LC resonant circuits and/or phase lock loops which are most commonly utilized in prior analogue tone detectors must be operated at unacceptably low response times in order to obtain sufficient selectivity for guarding against digit simulation by speech or noise introduced at the calling station. These analogue detectors tend to be excessively complex, difficult to miniaturize, and costly to manufacture. Furthermore, since the tone signals are processed entirely in the analogue form, the maintenance and adjustment of these systems tend to be difficult.
Digital multi-frequency or DTMF tone detection offers economy, performance, size and reliability benefits which appear capable of overcoming or substantially mitigating these and other problems associated with analogue detectors. Examples of prior digital multi-frequency tone decoders are disclosed in U.S. Pat. Nos. 3,537,001, 3,760,269, 3,790,720, and 3,917,912.